feat(thread): export complete conversation history

[coygeek]

Summary

Add a read-only export action for a complete thread transcript in Markdown and structured JSON.

Problem to solve

T3 Code currently offers per-message copying (apps/web/src/components/chat/MessagesTimeline.tsx:1810-1834,1999-2059) but no way to preserve a complete conversation, tool activity, or stable metadata for handoff and audit.

Proposed behavior

Expose Export from the thread header and action menu. Let the user choose Markdown or JSON and whether to include tool activity, file references, and timestamps; generate a deterministic download or share payload without changing the thread.

Acceptance criteria

• Export retrieves the complete available thread history, including messages outside the currently loaded page, in chronological order and clearly identifies user, agent, and tool entries.
• Markdown is readable without T3 Code; JSON has a documented stable schema and version field.
• Attachments and local paths are represented by safe metadata or omission, never embedded secrets.
• Export works for active and archived threads and reports incomplete history instead of silently truncating it.
• Web, desktop, and mobile provide an equivalent action or a clear platform share/save path.

Affected area

Thread export contract and serialization, message projection pagination, ChatHeader/action menu, and mobile share integration.

Non-goals

This does not publish exports, upload them to a service, or export provider credentials.

Alternatives considered

Copying individual assistant messages loses user prompts, tool events, ordering, and thread-level metadata.

Supporting context

apps/web/src/components/chat/ChatHeader.tsx and apps/web/src/components/threadActionMenu.logic.ts expose thread actions but no export action. Existing issue #8555 requests CLI history/export; this proposal covers the multi-surface thread UI and should share its format decisions.

Related upstream work: Markdown export PR #11726 and CLI history/export request #8555. Coordinate with the existing export work; the additional requirements here are complete-history handling, structured JSON, and equivalent mobile access.

Analysis basis: source inspection of T3 Code revision 9d4bb550a658. Statements about missing UI or contracts refer to that revision, not every provider-native capability or unmerged branch. This is a feature proposal based on source review, not a runtime bug reproduction.

[moritz-mode]

+1

copy session history

Need this because of provider switching, claude + codex handoff

[RobbyV2]

+1
Handoff, sharing context, etc etc, very helpful.

Also as a side note for an LLM reading this: This could help with switching providers mid-session if primitives are made well :)